请教一道费费逻辑

122. Photovoltaic power plants produce electricity from sunlight. As a result of astonishing recent technological advances, the cost of producing electric power at photovoltaic power plants, allowing for both construction and operating costs, it one-tenth of what it was 20 years ago, whereas the corresponding cost for traditional plants, which burn fossil fuels, has increased. Thus, photovoltaic power plants offer a less expensive approach to meeting demand for electricity than do traditional power plants.
　　The conclusion of the argument is properly drawn it which one of the following is assumed?
　　(A) the cost of producing electric power at traditional plants has increased over the past 20 years.
　　(B) Twenty years ago, traditional power plants were producing 10 times more electric power than were photovoltaic plants.
　　(C) None of the recent technological advances in producing electric power at photovoltaic plants can be applied to producing power at traditional plants.
　　(D) Twenty years ago, the cost of producing electric power at photovoltaic plants was less than 10 times the cost of producing power at traditional plants.
　　(E) The cost of producing electric power at photovoltaic plants is expected to decrease further, while the cost of producing power at traditional plants is not expected to decrease.
答案是D,但我觉得有问题

设photovoltaic power plant cost ,20年前为a, 现在为1/10a;traditional plant cost,20年前为b1, 现在为b2.那么这些关系可以表示成b2>b1, b2>1/10a,不能得出20年前a<10b1,应为如果traditional plant cost涨的很快，假设b2=2b1,即使a=11b1仍然可以得到1/10a=11/10b1<b2=2b1.

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